The present invention relates to a solid state image pick-up element comprising a static induction transistor (hereinafter, abbreviated as SIT) having a photoelectrically converting function and a charge amplifying function.
Regarding a conventional solid state image pick-up element, there have been proposed a charge transfer device such as CCD (charge coupled devices) and a MOS transistor, but these elements have various disadvantages such as leakage of charge during the charge transferring operation, a low sensitivity of light detection and a difficulty in increasing the integration density. Recently, in order to eliminate the disadvantages mentioned above, there has been proposed a solid state image pick-up element comprising a SIT.
FIG. 1 shows a typical solid state image pick-up element of gate-capacitance type utilizing a SIT. In FIG. 1, on an n.sup.+ substrate 1 is deposited an n.sup.- epitaxial layer 2 forming an n.sup.- channel region. The substrate 1 constitutes a source region of the SIT. In the channel region 2, are formed an n.sup.+ drain region 3 and a p.sup.+ signal storing gate region 4. Moreover, on the channel region 2 is formed an insulating layer 5, and on the signal storing gate region 4, is formed a transparent insulating layer 6 on which is transparent gate electrode 7 is arranged. Further, on the drain region 3, is formed a drain electrode 8. In this manner, the gate capacitance 9 consists signal storing gate region 4, transparent insulating layer 6 and transparent gate electrode 7.
In the solid state image pick-up element of gate-capacitance type, the channel region 2 has been already depleted when the element is in a steady state in which no light is made incident thereon. Therefore, current does not flow between the source and the drain regions, even if a forward bias is applied therebetween.
When light input 10 is made incident upon the gate region 4 through the transparent electrode 7 and the transparent insulating layer 6, electron-hole pairs are generated in the channel region 2. In this case, the electrons flow into the grounded source region 1 and the holes are stored in the signal storing gate region 4. In this manner, the gate capacitance 9 is charged so that a potential at the signal storing gate terminal 11 is increased by .DELTA.V.sub.G. If it is assumed that the gate capacitance 9 is C.sub.G and an amount of charge carriers stored in the signal storing gate region 4 in response to the incident light amount is Q.sub.L, the gate potential .DELTA.V.sub.G can be expressed by .DELTA.V.sub.G =Q.sub.L /C.sub.G. Moreover, if a gate readout pulse is supplied to the signal storing gate 11 after a lapse of a certain storing period, the gate potential becomes above that of the gate readout pulse by .DELTA.V.sub.G, and the potential difference between the signal storing gate region 4 and the drain region 3 is decreased. Therefore, since the depletion layer is decreased, the drain current corresponding to the incident light amount flows between the source region 1 and the drain region 3 and is derived from the drain terminal 12. Since the SIT has an amplifying function, the gate potential is increased by an amplification factor and therefore, the large drain current is obtained.
As mentioned above, the solid state image pick-up element of SIT type has various advantages such as a simple construction and an amplifying function. There have been proposed various methods for discharging the signal charge stored in the signal storing gate region 4. In one known method, the stored charge is discharged simultaneously with the readout operation by applying a readout pulse to the gate terminal 11, and in another known method, the stored charge flows away by means of a switching transistor as disclosed in Japanese Patent Application Laid-Open Publication No. 15,229/80. However, in the former method, there occurs an inherent problem in effecting the signal readout operation together with the reset operation, and further the reset operation might be affected by characteristics of the readout pulse. In the latter method, the reset operation can be achieved accurately, but it is difficult to attain a high integration density because the construction becomes very complicated.